Poorly prepared for your viva means you didn’t bother, or you didn’t take it seriously.
Poorly prepared for your viva means you relied on hope rather than work to get you through.
Poorly prepared means you didn’t find out about what was involved with the viva.
You didn’t ask graduates about their experiences to help set your expectations.
Poorly prepared means you didn’t read your thesis or practise or do anything really after submission.
Poorly prepared maybe even means that you just submitted any old thing and hoped it would pass!
The actions (or lack of actions) that could lead to someone being poorly prepared for their viva are pretty clear.
So are the actions that could help candidates be ready.